1. Field of the Invention
The present invention relates to ferroelectrics such as oxides having a layered crystal structure that belong to what is called the aurivillius crystallographic group and that tend to have strains in crystal lattices. The invention also relates to a memory device using such a ferroelectric and manufacturing methods of the ferroelectric and the memory device.
2. Description of the Related Art
Ferroelectrics have hysteresis in their electric field vs. polarization characteristic. Based on this fact, proposals for realizing, by utilizing ferroelectrics, nonvolatile memory devices that can hold data without being backed up by a power supply were made in 1960's. However, the attempts to develop such nonvolatile memory devices were stopped because at that time the ferroelectric thin film forming technology has not been established yet and there occurred various problems such as interference between memory cells. However, because of marked improvements in the thin-film technology that occurred thereafter, in recent years attempts to develop nonvolatile memory devices utilizing ferroelectrics have become active again. (For example, refer to C. Araujo, J. Scott, R. Goddfrey, and L. McMillan, Appl. Phys. Lett., 48 (1986) 1439; and W. Kinney, W. Shepherd, W. Miller, J. Evans, and R. Womack, IEDM Tech. Dig., (1987) 850.)
Among ferroelectrics to constitute nonvolatile memory devices, bismuth strontium tantalate (Bi.sub.2 SrTa.sub.2 O.sub.9 ; hereinafter referred to as "BiSTa") particularly attracts attention which belongs to what is called the aurivillius crystallographic group and is superior in fatigue characteristic. (For example, refer to C. A-Paz de Araujo, J. D. Cuchiaro, L. D. McMillan, M. C. Scott, and J. F. Scott, Nature, 374 (1995) 627; K. Amanuma, T. Hase, and Y. Miyasaka, Appl. Phys. Lett., 66 (1995), 221; and S. B. Desu and D. P. Vijay, Master Sci. and Eng., B32 (1995) 75.) The aurivillius crystallographic group includes crystals that are represented by a stoichiometric composition formula [Bi.sub.2 O.sub.2 ].sup.2+1 [Me.sub.m-1 R.sub.m O.sub.3m+ ].sup.2- where m is an integer of 2 or more, Me is at least one element selected from the group consisting of sodium (Na), potassium (K), calcium (Ca), barium (Ba), strontium (Sr), lead (Pb), and bismuth (Bi), R is at least one element selected from the group consisting of iron (Fe), niobium (Nb), tantalum (Ta), and tungsten (W).
Recently, there have been made reports of successes in producing a BiSTa thin film by MOCVD (metal organic chemical vapor deposition). (Refer to T. Ami, K. Hironaka, C. Isobe, N. Nagel, M. Sugiyama, Y. Ikeda, K. Watanabe, A. Machida, K. Miura, and M. Tanaka, Mater. Res. Soc. Symp. Proc., 415 (1996) 195; and T. Li, Y. Zhu, S. B. Desu, C-H. Peng, M. Nagata, Appl. Phys. Lett. 68 (1996) 616.)
However, examinations of the characteristics of actually produced BiSTa single crystals revealed that one crystal had portions exhibiting anisotropy and portions not exhibiting anisotropy though these portions did not have any difference in composition. Further, observations with a transmission electron microscope (TEM) showed that portions not exhibiting anisotropy had such large strains that the symmetry of crystal lattices was lost. Similar lattice strain was found in BiSTa polycrystalline thin films by using a TEM. That is, it was found that materials having a complex crystal structure such as BiSTa have a problem that they tend to have such large strains that the symmetry of crystal lattices is lost and hence tend to have portions not exhibiting anisotropy and portions exhibiting anisotropy but not showing superior characteristics.
For the above reasons, when a memory device is formed by using a ferroelectric having a complex crystal structure such as BiSTa, its characteristics deteriorate depending on the volume ratio of portions not exhibiting anisotropy and portions not showing superior characteristics. Further, the device characteristics vary depending on the proportion of portions not exhibiting anisotropy and portions not showing superior characteristics.